Method for producing aromatic nitro compounds

ABSTRACT

In the reaction of aromatic compounds with nitrating acids comprising HNO3 and, if appropriate, H2SO4 and/or H2O and/or H3PO4 to form aromatic nitro compounds, according to the invention an amount of from 0.5 to 20,000 ppm of one or more surface-active substances from the group of the anionic, cationic, zwitterionic or nonionic surface-active substances is added to the reaction mixture.

This application is a 371 of PCT/EP98/06688 filed Apr. 26, 2000.

BACKGROUND OF THE INVENTION

The present invention relates to a process for preparing aromatic nitrocompounds by reacting aromatic compounds with nitrating acids comprisingHNO₃ and, if appropriate, H₂SO₄ and/or H₃PO₄ and/or H₂O, the saidprocess being carried out in the presence of surface-active substances.

Nitro compounds of the most varied types are important intermediates forpreparing plastics, dyes, auxiliaries, pharmaceuticals and otherchemicals.

There is abundant technical and scientific literature, including patentliterature, on the preparation of nitro aromatic compounds by variousprocesses involving isothermic or adiabatic conditions, batchwise orcontinuous operation and various reactors. Products required in smallamounts are preferably prepared in batchwise operation, whereas massproducts, such as nitrobenzene, nitrotoluene and nitrochlorobenzene, arepreferably prepared by continuous operation. Suitable reactors forbatchwise operation are, in general, stirred tanks, whereas, forexample, tubular reactors are preferred for continuous operation. In thecase of products produced in great quantities, there has been no lack ofattempts to recover the considerable heat of reaction at a hightemperature level and to utilize it for other purposes, for example forconcentrating the waste acid. More recent and promising adiabaticoperations have been described, inter alia, in EP-A 668 263 and EP-A 675104. Whereas the processes of the EP patent applications just mentionedalready have a high level of successful resource utilization (highmaterial yields and high energy recovery), it is still important,especially for mass products, to attempt to increase resourceutilization even more.

In the context of the preparation of 1-nitroanthraquinone, JP 06/293709mentions the use of di-(2-ethylhexyl)-sulphosuccinic acid Na salt. Thisprocess is characterized by a purely organic reaction medium(1,2-dichlorethane) and the use of NO₂ or N₂O₄ in combination with SO₃as nitrating agent. The di-(2-ethylhexyl)-sulphosuccinic acid Na salt isemployed in an amount of 0.17 g, based on the nitrating agent,consisting of 4.1 g of SO₃ and 4 g of NO₂. The 1- and2-nitroanthraquinone yield obtained does not exceed that of otherexamples without the use of the di-(2-ethylhexyl)-sulphosuccinic acid Nasalt mentioned. In Ind. Eng. Chem. Res. 34 (1995), 4305, it is noted inthe context of an investigation of the role of surface reactions inheterogenic nitrations of aromatic compounds that addition ofamphiphilic impurities to the organic phase slows down the reaction;this effect was confirmed using cetyl-trimethylammonium bromide asdeactivator (p. 4305, left-hand column, below FIG. 15). In thispublication, the reaction conditions involve a mixed acid of 41.41% byweight of H₂SO₄, 1% by weight of HNO₃ and the remainder to 100% byweight of water. When the reaction is realized on an industrial scale,this phenomenon of reaction slow-down leads to a drastic reduction inthe space-time yield.

Surprisingly, it has now been found that, in contrast to theobservations in Ind. Eng. Chem. Res., a considerable increase inreaction rate and yield is obtained under the conditions according tothe invention described further below when surface-active substances areemployed. These surprising results have the following advantages: formixing the reaction mixture, the expenses required for apparatus arelower, thus reducing the investment costs of a nitration process. Thecheap surface-active substances which are prepared as detergents in massproduction can be used, in particular. The surface-active substances areemployed in the ppm range. It is possible to select surface-activesubstances having a wide range of properties, and it is likewisepossible to select a wide range of other reaction conditions.

DESCRIPTION OF THE INVENTION

The invention relates to a process for preparing aromatic nitrocompounds by reacting nitratable aromatic compounds with nitrating acidscomprising HNO₃ and, if appropriate, H₂SO₄ and/or H₃PO₄ and/or H₂O atnormal to elevated temperature with constant mixing of the aromaticcompounds and the nitrating acids, characterized in that the reactionmixture comprises one or more surface-active substances from the groupof the anionic, cationic, zwitterionic or nonionic surface-activesubstances in an amount of from 0.5 to 20,000 ppm.

Surface-active substances which are suitable for the process accordingto the invention can be from the group of the anionic, cationic,zwitterionic or nonionic surface-active substances. Anionicsurface-active substances are, for example, lignosulphonic acids,formaldehyde condensates with aromatically attached sulphonic acidgroups, protein condensates, alkanesulphonates, alkylarylsulphonates andalkyl sulphates. Cationic surface-active substances are, for example,the quaternary ammonium salts. Zwitterionic surface-active substancesare betaines and sulphobetaines. Nonionic surface-active substances arepolyethers which are formed by alkoxylation of compounds having a mobileH atom with ethylene oxide, propylene oxide or butylene oxide, or amixture of two or more of these. Compounds having a mobile H atom ofthis type are, for example, alcohols, alkylphenols, phenols,alkylamines, carboxylic acids and carboxamides. Such surfactants, theirstructure and their preparation are known to the person skilled in theart working in this field.

From among the surface-active substances mentioned, those from the groupof the anionic or cationic surface-active substances are preferablysuitable for use in the process according to the invention, particularlypreferably those from the group of the anionic surface-activesubstances. Very particularly preferably, these are alkanesulphonates oralkyl sulphates having 10 to 22 C atoms.

It is possible to use a mixture of one or more surface-activesubstances. The amount of surface-active substances in the reactionmixture at the reactor inlet, for example, is from 0.5 to 20,000 ppm,preferably from 1 to 2000 ppm, particularly preferably front 1 to 200ppm, very particularly preferably from 5 to 150 ppm.

Surface-active substances from the groups mentioned are suitable for theprocess according to the invention independently of the degree of theirstability. The following configurations are, for example, feasible here:

The surface-active substance or a mixture of two or more surface-activesubstances is stable and remains in the HNO₃-depleted waste acid andbecomes reuseable according to the invention during reconcentration andrecycling of the waste acid.

The surface-active substance or a mixture of two or more surface-activesubstances is stable under the reaction conditions according to theinvention, but migrates into the organic phase of the aromatic nitrocompound and is, during various washing and other treatment processes,removed from the process according to the invention and accordingly hasto be replaced, for example at the reactor inlet.

The surface-active substance or a mixture of two or more surface-activesubstances is not entirely stable under the process conditions accordingto the invention; it does, however, act in the sense according to theinvention during the nitration reaction, but has to be replaced to theextent of its degradation/its destruction.

The surface-active substances can be introduced into the reactionmixture in various ways: Thus, it is possible to feed the surface-activesubstances into the feed stream of the organic compounds to be nitratedand/or into the feed stream of the nitrating acid. It is also possibleto add the surface-active substances to the reaction mixture as aseparate feed stream, for example at the reactor inlet.

The nitration process according to the invention which is characterizedby the use of surface-active substances can otherwise be applied to allcustomary processes operating with nitrating acids of HNO₃ andoptionally H₂SO₄ and/or H₃PO₄ and/or H₂O. Thus, it is possible, forexample, to operate under adiabatic or isothermic conditions. Owing tothe possibility of energy recovery at a high level, preference is givenhere to adiabatic conditions. Furthermore, the reaction according to theinvention can be carried out continuously or batchwise. Since it is theaim to introduce even products with relatively low tonnages into themore favourable continuous operation, preference is given to thiscontinuous operation.

The process according to the invention can be carried out in allnitration reactors known to the person skilled in the art. Exampleswhich may be mentioned are: the completely back-mixed stirred tank bothfor batchwise nitrations and in the form of a continuous stirred tankfor continuous nitrations; a stirred-tank cascade of, for example, 2 to5 stirred tanks for continuous nitration; a tubular reactor as reactorfor continuous nitrations. All of the reactors mentioned, but inparticular the tubular reactor, can be equipped with flow spoilerplates, perforated metal sheets or static mixers.

Owing to the generally greater proportion by volume of the acid phasewith respect to the organic phase of the compound to be nitrated, theacid phase is present here as continuous phase, whereas the organicphase of the compound to be nitrated is dispersed in the continuousphase by means of stirrers or dispersion on perforated metal sheets. Thenitrating acid and the compound to be nitrated can be combined by simplyfeeding both substances via pipes to the reactor in which they are thendispersed in the abovementioned manner. However, preference is given tointroducing the aromatic compound to be nitrated via one or more nozzlesinto the nitrating acid, followed by redispersion by means of thestirring described or with the aid of perforated metal sheets, slits andsimilar devices.

Examples of aromatic compounds to be nitrated which may be mentionedare: benzene, toluene, o-, m- or p-xylene, chlorobenzene, bromobenzene,chlorotoluene, bromotoluene, o-, m-, p-dichlorobenzene, phenol,napthalene, methylnaphthalene, phenol and phenol derivatives andaromatic amines and derivatives thereof. Most of these substances areliquid under reaction conditions. In principle, aromatic compounds whichare solid under reaction conditions can also be employed in the processaccording to the invention; in such cases, an auxiliary solvent isemployed to obtain a liquid phase to be nitrated. Preferred aromaticcompounds which are nitrated according to the invention are benzene,toluene, chlorobenzene and o-dichlorobenzene.

The nitrating acid used for the nitration comprises HNO₃ and, ifappropriate, H₂SO₄ and/or H₃PO₄ and/or water. For aromatic compoundswhich are readily nitrated, for example for phenols, a nitrating acid isused which comprises HNO₃ and, if appropriate, H₂O. In cases of a mixednitrating acid (HNO₃, H₂SO₄ and, if appropriate, H₂O) the H₂SO₄ is onsome occasions completely or partially replaced by H₃PO₄ to influenceisomer distribution. Such nitrating acids may additionally comprise oneor more of the abovementioned surface-active substances. In mostindustrially relevant cases, the nitrating acid comprises HNO₃, H₂SO₄and, if appropriate, a remainder to 100% by weight of H₂O and, ifappropriate, one or more surface-active substances. The nitrating acidpreferably comprises H₂O. For nitrations which are carried outisothermally, the nitrating acids used in most cases comprise 20 to 40%by weight of HNO₃, 49 to 60% by weight of H₂SO₄ and 11 to 20% by weightof H₂O (Ullmanns Encyklopädie der technischen chemic, 4, Aufl., Vol. 17,p. 386 (1979)). For adiabatic processes, use is made of nitrating acidscomprising 1 to 8% by weight, preferably 2 to 6% by weight, particularlypreferably from 2.5 to 5% by weight, of HNO₃ and 56-85% by weight,preferably 64 to 79% by weight, of H₂SO₄. The remainder to 100% byweight is water. All percentages are based on the total weight of H₂SO₄,HNO₃ and H₂O.

The reactants are mixed in the wide range from 20 to 160° C. In a mannerknown to the person skilled in the art, aromatic compounds which aremore sensitive to undesirable subsequent nitration and oxidation aremixed in a lower section of this range, for example at from 20 to 110°C., preferably from 30 to 100° C., particularly preferably from 40 to90° C. One such sensitive aromatic compound is, for example, toluene. Inthe case of aromatic compounds which are less sensitive to multiplenitration and oxidation, mixing is carried out in an elevated section ofthe range mentioned, for example at from 60 to 160° C., preferably from70 to 140° C., particularly preferably from 80 to 120° C. Such lesssensitive aromatic compounds are, for example, chlorobenzene,bromobenzene, dichlorobenzenes. If the nitration is carried outisothermally, the mixing temperature is maintained by suitable coolingdevices. If the nitration is carried out adiabatically, the resultingexothermic heat of reaction is not dissipated, but remains in thereaction mixture and may serve, in a manner that is likewise known, forconcentrating the waste nitrating acid after phase separation. Suchconcentrating is generally carried out by flash evaporation of the wasteacid under reduced pressure. In many cases, the H₂SO₄ concentration inthis waste acid is re-established completely, and the waste acid canthen, after the HNO₃ that has been consumed has been replaced, be usedonce more as nitrating acid in the process according to the invention.However, in any case at least partial concentration of the H₂SO₄ in thewaste acid is achieved.

The molar ratio of the aromatic compound to be nitrated to HNO₃ in thenitrating acid is generally from 0.9 to 1.5:1. To minimize formation ofundesirable polynitrated aromatic compounds, the molar ratio of aromaticcompound to HNO₃ is preferably from 1.0 to 1.5:1, particularlypreferably from 1.03 to 1.3:1, very particularly preferably from 1.05 to1.2:1. However, if the aromatic nitro compounds obtainable according tothe invention are to be subjected to dinitration, the extended range,starting at 0.9 mol of aromatic compound to 1 mol of HNO₃, is alsopermissible.

The process according to the invention results in shorter reaction timesand higher yields of the desired aromatic nitro compound. Furthermore,the higher yields are associated with higher selectivity, i.e.suppression of undesirable by-products.

A specific variant of the nitration process according to the inventionin the presence of surface-active substances relates to the preparationof mononitrotoluenes.

The specific variant accordingly relates to a process for the continuousor batchwise preparation of mononitrotoluenes by reacting toluene withan HNO₃/H₂SO₄/H₂O mixture in the presence of from 0.5 to 20,000 ppm ofone or more surface-active substances with formation, essentially, ofthe mononitrotoluenes and reaction water, characterized by the steps

a) feeding of the reaction participants toluene, HNO₃, H₂SO₄, H₂O andsurface-active substances in any sequence into a reactor equipped withmixing elements, in which

a1) the amount of HNO₃ is 1-8% by weight, the amount of H₂SO₄ is 56 to85, preferably 58 to 74% by weight and the amount of H₂O is theremainder to 100% by weight and 100% by weight signifies the sum ofHNO₃+H₂SO₄+H₂O,

a2) the H₂O is used as such, as dilution H₂O of the HNO₃, as dilutionH₂O of the H₂SO₄ or in a plurality of the said forms and

a3) the molar ratio of toluene to HNO₃ is 0.9-1.5,

b) rapid and intensive mixing of the totality of the reactionparticipants, using a mixing energy of 1 to 80 watts per liter of thetotal reaction mixture, preferably 1 to 70 W/l, particularly preferably1 to 60 W/l, very particularly preferably 5 to 50 W/l,

c) carrying out the reaction under adiabatic conditions, the reactionparticipants being fed in at temperatures such that the mixing proceedsin the range from 20-120° C., preferably from 30-110° C., particularlypreferably from 40-100° C., and the temperature at the end of tiereaction does not exceed 135° C.,

d) separating the reaction mixture, after carrying out the reaction,into an organic and an inorganic phase and

e) work-up of the substantially HNO₃-free inorganic phase bydistillation with removal of water, where the inorganic phase, ifappropriate, comprises the surface-active substance(s).

These variants are carried out batchwise or continuously, preferablycontinuously.

The continuous procedure can be carried out, for example, in thefollowing manner: the reaction participants are rapidly mixed in theirtotal amount in a mixing element and fed into a reactor as a mixture.The mixing time with the continuous procedure is generally less than 3sec., for example 1 msec. to 2.99 sec., preferably 1 msec. to 2 sec. Thereactor is insulated if required, substantially prevents back-mixing andis operated adiabatically. For the substantial prevention ofback-mixing, the reactor is subdivided or is composed of a plurality ofchambers or units; at the transitions between the reactor parts, thereaction mixture is redispersed. The mixture reacted to exhaustion runsoff and is separated in a separation vessel; the separation proceedsrapidly. The organic phase is worked-up in a conventional manner, e.g.by washing and distillation, or is immediately fed to a secondnitration. Generally, in particular when there is an excess of toluene,the inorganic phase separated off is virtually free of nitric acid. Ifthis is not the case, in particular when there is an excess of nitricacid, residual nitric acid can be consumed in a post-reactor withaddition of further toluene in the sense of a reactive extraction. Theinorganic acid phase substantially freed of nitric acid is preferablyfed to a flash evaporation with utilization of the heat of reactionabsorbed and under reduced pressure. In this case, water is removed fromthe acid and, preferably, simultaneously, the acid is brought to theinput concentration and the input temperature. This acid is then, asH₂SO₄, directly suitable for use in step a) and comprises, ifappropriate, the surface-active substance(s). This return of theworked-up inorganic phase (H₂SO₄, H₂O) to the process results in acirculation procedure for the H₂SO₄ and, if appropriate, thesurface-active substance(s); it can be expedient to eject a small partof this H₂SO₄ to keep any contamination to a low level. In the eventthat the inorganic phase still contains toluene, nitrotoluene and anyorganic by-products, it can be expedient to strip the inorganic phasebefore the flash evaporation to remove the organic compounds. The waterobtained subsequently as flash condensate is then of higher purity andits disposal is simpler. Obviously, the flash condensate can also befreed of organic compounds, e.g. by stripping or phase separation, aresidual flash condensate and a high-purity water-acid phase similarlyremaining. The organic compounds arising in the post-reaction of theHNO₃ with further toluene and in the stripping or other separations,such as phase separation, can be added to the process at a suitablepoint (toluene, (di)nitrotoluene) or are ejected and disposed of(impurities, by-products).

The reaction participants can be fed to the reactor equipped with mixingelements together, but also individually or as mixtures of two or threethereof simultaneously or successively. The feedstocks can be mixed, forexample, in such a way that toluene and nitric acid or, it required,water are simultaneously or successively added as separate streams tothe concentrated recycled sulphuric acid, in which case the nitric acidcan be diluted by water and/or sulphuric acid and water. Toluene canalso be premixed with water and sulphuric acid and the resultingemulsion is further intensively mixed with nitric acid which can bemixed with sulphuric acid and/or water. Furthermore, the toluene canalso be intensively mixed with a nitrating acid of sulphuric acid,nitric acid and water and then further treated according to theinvention. The surface-active substance(s) to be employed according tothe invention can be added to any of these streams or stream mixtures oremployed separately. Still other variants of the feeding of the reactionparticipants, their intensive mixing and further treatment are easilyrecognizable to the person skilled in the art. For this purpose, mixingelements known in the art are suitable, e.g.: 1. static mixers, 2.pumps, 3. nozzles, 4. agitators or combinations thereof.

For the reaction to succeed, it is of little importance in whichsequence and combination the reaction participants nitric acid andtoluene as well as sulphuric acid and water and the surface-activesubstance(s) are mixed together, as long as the reaction mixture has thecomposition according to the invention after the total mixing and themixing takes place at the intensity according to the invention and, whenthe reaction is carried out continuously, substantially free fromback-mixing.

The mixing intensity, in the case of the batchwise procedure, apart fromthe high energy input, can also be characterized by the short reactionparticipant addition time which is 0.001 to 15%, preferably 0.001 to 3%,of the time which is required for the course of the reaction betweentoluene and nitric acid. It is thus also possible to carry out theprocess according to the invention batchwise in a stirred tank.

The feeding and intensive mixing of the reaction participants arefollowed, in the continuous procedure, by at least two redispersions.For this purpose, in the reactor there are present, preferably insections, static mixer elements, if required also in the form ofspherically shaped fixed internals, such as perforated metal sheets,slotted metal sheets, impact baffles, vanes or agitators or similarinternals or elements known for this purpose to the person skilled inthe art.

Continuously operated reactors for the specific variant which can bementioned by way of example are as follows: tubular reactors havinginternals for redispersion, such as vanes, deflection baffles, staticmixers or agitators and the like; intensively stirred tanks in a cascadearrangement; loop reactors having internals as above; combinations of aplurality of the said apparatuses; other reactors of equivalent action,such as chamber reactors with agitators in each chamber. Tubularreactors having internals are preferably used. The internals arepreferably perforated metal sheets. All internals represent subdivisionsof the entire apparatus which equally serve for the redispersion and thesubstantial prevention of back-mixing.

After the intensive mixing, after each dispersion or after the mixturehas flowed through a certain part-length of the reactor, coalescence ofthe dispersion droplets is observed which can be reversed byredispersion. The number of redispersion operations is, according to theinvention, 2 to 50, preferably 3 to 30, particularly preferably 4 to 20.To overcome the pressure drops occurring in this case, a mixing energyof 1 to 80 watts/liter, preferably 1 to 70 W/l, particularly preferably1 to 60 W/l, very particularly preferably 5 to 50 W/l, per liter of thetotal reaction mixture is added to the reaction system with the reactionparticipants.

The reaction participants are mixed in the specific variant in the rangefrom 20 to 110° C., preferably from 30 to 110° C., particularlypreferably from 40 to 110° C. Adiabatic reaction conditions aremaintained. The final temperature is dependent on the height of themixing temperature, on the ratios of the amounts of the reactionparticipants and on the conversion rate; it generally does not exceed135° C. and usually does not exceed 125° C.

The content of added nitric acid in the reaction mixture at the time ofmixing in the specific variant, based oil the sum of nitric acid,sulphuric acid and water, is 1 to 8% by weight, preferably 1 to 6% byweight, particularly preferably 1.5 to 4% by weight. Nitric acid can beused in highly concentrated form or as an azeotrope, but preferably inthe form of the inexpensive “weak acid”, having approximately 60-65% byweight.

The content of sulphuric acid in the reaction mixture at the time ofmixing in the specific variant, based on the sum of nitric acid,sulphuric acid and water, is 56-85% by weight, preferably 58-74% byweight, particularly preferably 60-72% by weight, very particularlypreferably 61-69% by weight. These figures do not include anyprocess-specific impurities which may be contained in the event of anH₂SO₄ circulation procedure.

The amount of one or more surface-active substances is that specifiedabove. The remainder to 100% by weight is H₂O. This can be used as such,as dilution H₂O of the H₂SO₄, as dilution H₂O of the HNO₃ or in aplurality of the said forms. H₂O is preferably present as dilution H₂Oof both the H₂SO₄ and of the HNO₃.

Since the intensity of nitration with changing contents of nitric acidin the nitrating acid is dependent on the ratio of sulphuric acid towater, it is determined and, if required, adjusted on the basis of thesulphuric acid concentration of the outflowing and substantially nitricacid-free spent acid. This H₂SO₄ concentration of the spent acid is tobe, according to the invention, 62 to 74% by weight, preferably 64 to72% by weight, particularly preferably 66 to 70% by weight. For reuse,the outflowing sulphuric acid is concentrated by 0.6-7 percentagepoints, in many cases by 1.5-3 percentage points, water (reaction water,possibly dilution water) being ejected by distillation. For this purposethe heat of reaction absorbed from the outflowing H₂SO₄ owing to theadiabatic reaction conditions is preferably utilized and reducedpressure in the range from 1 to 100 mbar, preferably from 5-80 mbar,particularly preferably from 10-75 mbar, is employed. This can becarried out, for example, in the form of a flash evaporation. The H₂SO₄recovered in this case is suitable for use in step a). The ejection ofwater by distillation is preferably carried out in such a way that thetemperature and concentration of the concentrated H₂SO₄ are directlyequivalent to the values demanded in step a). Such a utilization of theheat of reaction makes the process according to the invention moreeconomical than the known processes for the preparation ofnitrotoluenes.

Possible embodiments with respect to the nitrating acids having varyingcompositions, to outflowing H₂SO₄ concentrations, temperature conditionsand pressure of the flash evaporation and degree of concentration of theH₂SO₄ may be summarized by way of example as follows, without mentioningthe surface-active substance(s), (cases a, b and c):

Nitrating acid a b c HNO₃ (% by weight) 4.00 3.00 2.50 H₂SO₄ (% byweight) 64.11 65.56 66.79 H₂O (% by weight) 31.89 31.44 30.71 Strengthof the acids used HNO₃ (% by weight) 60.0 60.0 60.0 H₂SO₄ (% by weight)68.69 69.01 69.69 Outflowing H₂SO₄ (% by weight) 66.0 67.0 68.0 Mixingtemperature (° C.) 80 85 90 Final temperature (° C.), approximately 120115 115 Pressure in flash evaporation 40 50 60 (approximate mbar)

The molar ratio of toluene to HNO₃ is generally 0.9-1.5. In order tominimize the formation of undesirable dinitrotoluenes, the molar ratioof toluene to nitric acid is preferably 1.0 to 1.5, particularlypreferably 1.03 to 1.3, very particularly preferably 1.05 to 1.2.However, if the nitrotoluenes available according to the invention areto be fed to the dinitration, other molar ranges, e.g. 0.9-1.2 mol,preferably 0.9-1.05 mol, particularly preferably 0.95-1 mol, of tolueneper mole of nitric acid are also permissible.

The reaction of the process according to the invention proceedsaccording to the formula:

C₆H₅—CH₃+HNO₃→O₂N—C₆H₄—CH₃+H₂O

Thus toluene and HNO₃ are introduced into the process andmononitrotoluene and reaction water are ejected, while the H₂SO₄/H₂Omixture described, which, if appropriate, contains the surface-activesubstance(s), represents the reaction medium.

Since, when the process is carried out industrially, dilute nitric acidsare advantageously used, depending on the prices of the nitric acidsrespectively available, additionally to the reaction water, dilution H₂Oof the HNO₃ must also be ejected.

The organic phase arising in the separation of the reaction mixture canbe worked up to give pure mononitrotoluene or be fed to thedinitrotoluene preparation. In the former case at least molar amounts oftoluene or a slight molar excess is used, as described above, in ordernot only to consume the HNO₃ but also to repress the second nitration;any toluene excess is distilled off from the organic phase separatedoff. Before this, the organic phase can be washed in order to separateoff water-, acid- or alkali-soluble impurities, such as inorganic andorganic acids and phenolic impurities. However, the formation ofoxidation products (phenols, oxidation of the CH₃ group) is stronglysuppressed in the process according to the invention. Likewise, theformation of dinitrotoluenes is highly repressed. However, thesedinitrotoluenes are not an interference if a second nitration is in anycase intended; therefore, in such cases, the procedure may also becarried out with a toluene deficiency.

A further specific variant of the nitration process according to theinvention in the presence of surface-active substances relates to thepreparation of mononitrohalogenobenzenes.

The second specific variant accordingly relates to a process for thecontinuous or batchwise preparation of mononitrohalogenobenzenes byreacting halogenobenzenes with an HNO₃/H₂SO₄/H₂O mixture in the presenceof from 0.5 to 20,000 ppm of one or more surface-active substances withformation, essentially, of the mononitrohalogenobenzenes and reactionwater, characterized by the steps

a) feeding of the reaction participants halogenobenzene, HNO₃, H₂SO₄ andH₂O in any sequence into a reactor equipped with mixing elements, inwhich

a1) the amount of HNO₃ is from 1 to 8% by weight, the amount of H₂SO₄ is56 to 85% by weight and the amount of H₂O is the remainder to 100% byweight and 100% by weight signifies the sum of HNO₃+H₂SO₄+H₂O,

a2) the H₂O is used as such, as dilution H₂O of the HNO₃, as dilutionH₂O of the H₂SO₄ or in a plurality of the said forms and

a3) the molar ratio of halogenobenzene to HNO₃ is 0.9-1.5,

b) rapid and intensive mixing of the totality of the reactionparticipants, using a mixing energy of 1 to 80 watts per liter of thetotal reaction mixture, preferably 1 to 70 W/l, particularly preferably1 to 60 W/l, very particularly preferably 5 to 50 W/l,

c) carrying out the reaction under adiabatic conditions, the reactionparticipants being fed in at temperatures such that the mixing proceedsin the range from 60 to 160° C. and the temperature at the end of thereaction does not exceed 180° C.,

d) separating the reaction mixture, after carrying out the reaction,into an organic and an inorganic phase and

e) work-up of the substantially HNO₃-free inorganic phase bydistillation with removal of water, where the inorganic phase, ifappropriate, comprises the surface-active substance(s).

For the purpose of the invention, halogenobenzenes are chlorobenzene,o-, m-, p-dichlorobenzene, o-, m-, p-chlorotoluene and bromobenzene,preferably chlorobenzene and o-, m-, p-dichlorobenzene, particularlypreferably chlorobenzene and o-dichlorobenzene.

This variant, too, can be carried out continuously or batchwise,preferably continuously. For continuous operation, the procedure of thefirst specific variant can be followed, using halogenobenzene instead oftoluene.

In the second specific variant, the reaction participants are mixed inthe range from 60 to 160° C., preferably from 70 to 140° C.,particularly preferably from 80 to 120° C. Adiabatic reaction conditionsare maintained. The final temperature is dependent on the height of themixing temperature, on the ratios of the amounts of the reactionparticipants and on the conversion rate; it generally does not exceed180° C. and usually does not exceed 160° C.

The content of added nitric acid in the reaction mixture at the time ofmixing, based on the sum of nitric acid, sulphuric acid and water, is 1to 8% by weight, preferably 2 to 6% by weight, particularly preferably2.5 to 5% by weight, in the second specific variant.

The content of sulphuric acid in the reaction mixture at the time ofmixing, based on the sum of nitric acid, sulphuric acid and water, is 56to 85% by weight, preferably 56.5 to 84.5% by weight, particularlypreferably 65 to 79% by weight, very particularly preferably 67.5 to 77%by weight, in the second specific variant.

The remainder to 100% is H₂O.

According to the invention, the H₂SO₄ concentration of the spent acid inthe second specific variant should be 60 to 85% by weight, preferably 68to 80% by weight, particularly preferably 70 to 78% by weight. Forreuse, the outflowing sulphuric acid is concentrated by 0.6 to 7.5percentage points, in many cases by 1.7 to 4.2 percentage points. Forthis purpose the heat of reaction absorbed from the outflowing H₂SO₄ isutilized and reduced pressure, for example from 40 to 150 mbar,preferably from 40 to 120 mbar, particularly preferably from 50-100mbar, is employed. Here, too, this can be carried out, for example, inthe form of a flash evaporation.

Possible embodiments with respect to the nitrating acids having varyingcompositions, to outflowing H₂SO₄ concentrations, temperature conditionsand pressure of the flash evaporation and degree of concentration of theH₂SO₄ may be summarized by way of example for the second specificvariant as follows, likewise without mentioning the surface-activesubstance(s) (cases a and c: chlorobenzene; case b: o-dichlorobenzene):

Nitrating acid a b c HNO₃ (% by weight) 3.00 3.00 5.00 H₂SO₄ (% byweight) 68.50 74.37 67.50 H₂O (% by weight) 28.50 22.63 27.50 Strengthof the acids used HNO₃ (% by weight) 60.00 60.00 60.00 H₂SO₄ (% byweight) 72.11 78.28 73.64 Outflowing H₂SO₄ (% by weight) 70.00 76.0070.00 Mixing temperature (° C.) 110 110 100 Final temperature(approximately ° C.) 140 140 150 Pressure in flash evaporation 95 48 60(approximately mbar)

The molar ratio of halogenobenzene to HNO₃ is generally 0.9 to 1.5. Inorder to minimize the formation of undesirable dinitrohalogenobenzenes,the molar ratio of halogenobenzene to nitric acid is preferably 1.0 to1.5, particularly preferably 1.01 to 1.3, very particularly preferably1.05 to 1.2. However, if the nitrohalogenobenzenes available accordingto the invention are to be fed to the dinitration, other ranges, e.g.0.9 to 1.2 mol, preferably 0.9 to 1.05 mol, particularly preferably 0.95to 1 mol, of halogenobenzene per mole of nitric acid are alsopermissible.

The reaction of the process according to the invention proceedsaccording to the formula:

C₆H₅—Ha1+HNO₃→O₂N—C₆H₄—Ha1+H₂O.

The organic phase arising in the separation of the reaction mixture canbe worked analogously to the first specific variant.

The invention is further described in the following illustrativeexamples in which all parts and percentages are by weight unlessotherwise indicated.

EXAMPLE 1

At 75° C., a stream consisting of 187.8 kg of H₂SO₄ (70%)/h and 8.7 kgof HNO₃ (67%)/h and a stream of 9.4 kg of toluene/h were fedsimultaneously into a tubular reactor having perforated plates asredispersing elements. The nitrating acid contained 25 ppm ofalkanesulphonate (C₁₂-C₁₈). After a residence time of about 35 sec., themixture reacted to exhaustion left the reactor. Phase separation gave:

Organic phase: 12.92 kg/h of the following composition (calibrated GC):Toluene: 6.41% o-Nitrotoluene: 54.06% m-Nitrotoluene: 5.36%p-Nitrotoluene: 33.38% 2,4-Dinitrotoluene: 0.19% 2,6-Dinitrotoluene:0.07% Dinitro-o-cresol: 0.14% Dinitro-p-cresol: 0.40% Acid phase: 121ltr/h with 4.30 g of mononitrotoluenes per ltr of acid phase.

This corresponds to a yield of mononitrotoluenes of 98.6% of thetheoretical yield.

EXAMPLE 2 Comparative Example

At 75° C., a stream consisting of 187.8 kg of H₂SO₄ (70%)/h and 8.7 kgof HNO₃ (67%)/h and a stream of 9.4 kg of toluene/h were fedsimultaneously into a tubular reactor having perforated plates asredispersing elements; alkanesulphonate was not employed. After aresidence time of about 35 sec., the mixture reacted to exhaustion leftthe reactor. Phase separation gave:

Organic phase: 12.40 kg/h of the following composition (calibrated GC):Toluene: 15.54% o-Nitrotoluene: 48.65% m-Nitrotoluene: 4.91%p-Nitrotoluene: 30.00% 2,4-Dinitrotoluene: 0.29% 2,6-Dinitrotoluene:0.11% Dinitro-o-cresol: 0.10% Dinitro-p-cresol: 0.39% Acid phase: 121ltr/h with 3.80 g of mononitrotoluenes per ltr of acid phase.

This corresponds to a yield of mononitrotoluenes of 85.4% of thetheoretical yield.

EXAMPLE 3

At 75° C., a stream consisting of 187.8 kg of H₂SO₄ (70%)/h and 8.7 kgof HNO₃ (67%)/h and a stream of 9.4 kg of toluene/h were fedsimultaneously into a tubular reactor having perforated plates asredispersing elements. The nitrating acid contained 33 ppm ofbenzyltrimethylammonium chloride. After a residence time of about 35sec., the mixture reacted to exhaustion left the reactor. Phaseseparation gave:

Organic phase: 12.80 kg/h of the following composition (calibrated GC):Toluene: 7.00% o-Nitrotoluene: 54.02% m-Nitrotoluene: 5.39%p-Nitrotoluene: 32.91% 2,4-Dinitrotoluene: 0.15% 2,6-Dinitrotoluene:0.05% Dinitro-o-cresol: 0.12% Dinitro-p-cresol: 0.36% Acid phase: 121ltr/h with 4.20 g of mononitrotoluenes per ltr of acid phase.

This corresponds to a yield of mononitrotoluenes of 97.3% of thetheoretical yield.

EXAMPLE 4

At 110° C., a stream consisting of 187.8 kg of H₂SO₄ 70%/h and 8.7 kg ofHNO₃ 67%/h and a stream of 11.5 kg of chlorobenzene/h were fedsimultaneously into a tubular reactor equipped with perforated plates asredispersing elements. The nitrating acid contained 100 ppm of alkanesulphonate. After a residence time of about 35 sec., the mixture reactedto exhaustion left the reactor.

Phase separation gave:

Organic phase: 14.30 kg/h of the following composition (calibrated GC):Chlorobenzene 7.20% o-Nitrotoluene: 36.19% m-Nitrotoluene: 1.64%p-Nitrotoluene: 54.97% Acid phase: 121 ltr/h with 5.4 g ofmononitrochlorobenzenes per ltr of acid phase.

This corresponds to a yield of mononitrochlorobenzenes of 98.2% of thetheoretical yield.

EXAMPLE 5 Comparative Example

At 110° C., a stream consisting of 187.8 kg of H₂SO₄ 70%/h and 8.7 kg ofHNO₃ 67%/h and a stream of 11.5 kg of chlorobenzene/h were fedsimultaneously into a tubular reactor equipped with perforated plates asredispersing elements; alkane sulphonate was not employed. After aresidence time of about 35 sec., the mixture reacted to exhaustion leftthe reactor.

Phase separation gave:

Organic phase: 13.9 kg/h of the following composition (calibrated GC):Chlorobenzene 15.53% o-Nitrotoluene: 32.86% m-Nitrotoluene: 1.47%p-Nitrotoluene: 50.14% Acid phase: 121 ltr/h with 5.1 g ofmononitrochlorobenzenes per ltr of acid phase.

This corresponds to a yield of mononitrochlorobezenes of 87.3% of thetheoretical yield.

EXAMPLE 6

At 75° C., a stream consisting of 187.8 kg of H₂SO₄ 70%/h and 8.7 kg ofHNO₃ 67%/h and a stream of 9.4 kg of toluene/h were fed simultaneouslyinto a tubular reactor equipped with perforated plates as redispersingelements. The mixed acid contained 25 ppm of alkyl sulphate. After aresidence time of about 35 sec., the mixture reacted to exhaustion leftthe reactor.

Phase separation gave:

Organic phase: 12.85 kg/h of the following composition (calibrated GC):Toluene: 6.57% o-Nitrotoluene: 53.94% m-Nitrotoluene: 5.35%p-Nitrotoluene: 32.30% 2,4-Dinitrotoluene: 0.25% 2,6-Dinitrotoluene:0.11% Dinitro-o-cresol: 0.12% Dinitro-p-cresol: 0.36% Acid phase: 121ltr/h with 4.20 g of mononitrotoluenes per ltr of acid phase.

This corresponds to a yield of mononitrotoluenes of 98.5% of theory.

Although the present invention has been described in detail withreference to certain preferred versions thereof, other variations arepossible. Therefore, the spirit and scope of the appended claims shouldnot be limited to the description of the versions contained therein.

What is claimed is:
 1. Process for preparing aromatic nitro compoundscomprising the step of reacting a nitratable aromatic compound withnitrating acids comprising HNO₃ at a normal to elevated temperature withconstant mixing of the aromatic compound and the nitrating acids,wherein the reaction mixture comprises one or more surface-activesubstances comprising a member selected from the group consisting ofanionic, cationic, zwitterionic nonionic surface-active substances in anamount of from 0.5 to 20,000 ppm.
 2. Process according to claim 1,wherein the content of one or more surface-active substances is from 1to 2000 ppm.
 3. Process according to claim 1, wherein the one or moresurface-active substances employed comprise a member selected from thegroup consisting of anionic and cationic surface-active substances. 4.Process according to claim 1, wherein the reaction is carried out underadiabatic or isothermic conditions.
 5. Process according to claim 1,wherein the reaction is carried out continuously or batchwise. 6.Process according to claim 1, wherein the reactor used for the reactionis a stirred tank, a stirred tank cascade or a tubular reactor, whichcan be equipped with flow spoiler plates, perforated metal sheets orstatic mixers.
 7. Process according to claim 1, wherein the aromaticcompound is introduced via one or more nozzles into the nitrating acid.8. Process according to claim 1, wherein the aromatic compound reactedcomprises a member selected from the group consisting of benzene,toluene, o-xylene, m-xylene, p-xylene, chlorobenzene, bromobenzene,chlorotoluene, bromotoluene, o-dichlorobenzene, m-dichlorobenzene, andp-dichlorobenzene, naphthalene, methylnaphthalene, phenol, phenolderivatives, aromatic amines and derivatives.
 9. The method of claim 1,wherein the step of reacting the nitratable aromatic compound with thenitrating acids is carried out with a member selected from the groupconsisting of H₂SO₄, H₂O and H₃PO₄.
 10. Process for the continuous orbatchwise preparation of mononitrotoluenes by reacting toluene with anHNO₃/H₂SO₄/H₂O mixture in the presence of from 0.5 to 20,000 ppm of oneor more surface-active substances with formation, essentially, of themononitrotoluenes and reaction water the process comprising the steps ofa) feeding of the reaction participants toluene, HNO₃, H₂SO₄, H₂O andsurface-active substances in any sequence into a reactor equipped withmixing elements, in which a1) the amount of HNO₃ is 1-8% by weight, theamount of H₂SO₄ is 56 to 85% by weight and the amount of H₂O is theremainder to 100% by weight and 100% by weight signifies the sum ofHNO₃+H₂SO₄+H₂O, a2) the H₂O is used as such, as dilution H₂O of theHNO₃, as dilution H₂O of the H₂SO₄ or in a plurality of the said formsand a3) the molar ratio of toluene to HNO₃ is 0.9-1.5, b) rapid andintensive mixing of the totality of the reaction participants, using amixing energy of 1 to 80 watts per liter of the total reaction mixture,c) carrying out the reaction under adiabatic conditions, the reactionparticipants being fed in at temperatures such that the mixing proceedsin the range from 20-120° C. and the temperature at the end of thereaction does not exceed 135° C., d) separating the reaction mixture,after carrying out the reaction, into an organic and an inorganic phaseand e) work-up of the substantially HNO₃-free inorganic phase bydistillation with removal of water, where the inorganic phase, ifappropriate, comprises the surface-active substances.
 11. Process forthe continuous preparation of mononitrohalogenobenzenes by reactinghalogenobenzenes with an HNO₃/H₂SO₄/H₂O mixture in the presence of from0.5 to 20,000 ppm of one or more surface-active substances withformation, essentially, of the mononitrohalogenobenzenes and reactionwater, the process comprising the steps of a) feeding of the reactionparticipants halogenobenzene, HNO₃, H₂SO₄, H₂O and surface-activesubstances in any sequence into a reactor equipped with mixing elements,in which a1) the amount of HNO₃ is from 1 to 8% by weight, the amount ofH₂SO₄ is 56 to 85% by weight and the amount of H₂O is the remainder to100% by weight and 100% by weight signifies the sum of HNO₃+H₂SO₄+H₂,a2) the H₂O is used as such, as dilution H₂O of the HNO₃, as dilutionH₂O of the H₂SO₄ or in a plurality of the said forms and a3) the molarratio of halogenobenzene to HNO₃ is 0.9-1.5, b) rapid and intensivemixing of the totality of the reaction participants, using a mixingenergy of 1 to 80 watts per liter of the total reaction mixture, c)carrying out the reaction under adiabatic conditions in reactors whichsubstantially prevent the back-mixing of the reaction participants andin which the reaction participants are redispersed at least 2 timeswhilst flowing through the reactor, the reaction participants being fedin at temperatures such that the mixing proceeds in the range from 60 to160° C. and the temperature at the end of the reaction does not exceed180° C., d) separating the reaction mixture, after carrying out thereaction, into an organic and an inorganic phase and e) work-up of thesubstantially HNO₃-free inorganic phase by distillation with removal ofwater, where the inorganic phase, if appropriate, comprises thesurface-active substances.